Ian_Rob
Well-known member
To check my ST60+ can I just compare the indicated SOG withone of the GPS apps that are available , whilst motoring at slack tide, to get an accurate calibration?
Calibrating Speed on a ST60+
- Thread starter Ian_Rob
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john_morris_uk
Well-known member
Raymarine ST60 actually have a quick and dirty calibration method where the log is matched to SOG at the press of a button.
I’ve used it but the last time we calibrated we used a Walker Trailing Log as a reference and went through the full calibration process. If the spinner is clean and the waves/swell not too big, Walker logs are remarkably accurate.
I’ve used it but the last time we calibrated we used a Walker Trailing Log as a reference and went through the full calibration process. If the spinner is clean and the waves/swell not too big, Walker logs are remarkably accurate.
jac
Well-known member
Yes you can but accuracy is limited. A better approach is to find a convenient measured mile and use that. Motor at 6 knots and that measured mile should take you 10 minutes exactly. repeat in the opposite direction and then take the correction from the Average of the two. So if the average is 12 minutes then you were actually doing 5 knots
Ian_Rob
Well-known member
OK. I have just looked it up and I think there is measured mile on the west side of Southampton Water (which isn’t far) so I will probably try the more accurate method. Thank you.
earlybird
Well-known member
I've spent time doing this. It's unlikely that the ST60 is linear throughout its range. Mine seemed to underread at the lower end and overread at high speeds. There are also likely to be differences depending on which tack the boat is sailing on, different again if motoring. Like life, it all need compromise.
The measured distance that I used occasionally was off Arran, in the Clyde but eventually I decided that comparing with GPS over shorter distances was much easier and just as useful.
The measured distance that I used occasionally was off Arran, in the Clyde but eventually I decided that comparing with GPS over shorter distances was much easier and just as useful.
yotter
Active member
I agree with earlybird and find that my ST60, or probably the Airmir padddlewheel is non linear, it reads low at low speed and high at higher speeds. So, I use the quick and dirty method and match to SOG at around 5 or 6 knots, in a location with slack water (a canal is ideal
) That way it is reasonably accurate at average passage speeds. ```
) That way it is reasonably accurate at average passage speeds. ```
johnalison
Well-known member
You are very unlikely to get an accurate calibration in one go, and certainly not one that covers a range of speeds, sea states and sail/motor. I set mine when motoring at 6+ knots and then aim to refine this in the light of experience, in the hope that it will be about right when sailing or motoring at normal speeds. As said, logs tend to under-read at lower speeds, so some mental adjustment can be necessary. A clean log impeller in good condition should read down to 0.1 knot, which can be a useful check. Set to GPS will probably not be exactly right but will give a useful approximation to start with.
jwilson
Well-known member
ST60s etc have a "match GPS speed" function in the user menus. Motor at about 4 or 5 knots on steady throttle for 150-200 metres in a minimal tide area or time, turn and repeat on the reciprocal course, then if the GPS speeds each way are within 0.1 or 0.2 kt of each other do a third run and set your log to match the GPS. It will be about as good as you will get for log accuracy. As others have said logs are rarely accurate throughout the speed range, hence do it at mid-speed.
The trouble with measured mile markers is that there may not be one close, also they often tend to be in open water and sea conditions can affect your steady speed. You can usually find a smaller area with minimal tidal flow or pick a time of slack water for the quick 'match-GPS' way.
The trouble with measured mile markers is that there may not be one close, also they often tend to be in open water and sea conditions can affect your steady speed. You can usually find a smaller area with minimal tidal flow or pick a time of slack water for the quick 'match-GPS' way.
john_morris_uk
Well-known member
Send reply #2
Constant speed is not critical.
westernman
Well-known member
An ST60 type log will never be accurate. But it is still useful as immediate feedback as to whether a small tweak on a sheet increased speed or decreased it.
jwilson
Well-known member
The constant engine speed is to see if your two-way run differs on GPS. If so either wind/current/tide is affecting the SOG vs STW.
I agree though, STW or log speed is not that important unless you are navigating by dead reckoning (maybe 0.01% of users now) or you think you can improve your speed sailing by tweaking sheets or other trim. As a very long-term dinghy sailor I can only answer that dinghies don't have logs and a good sailor will always beat a poor one without a log. For serious top-end big-boat racers who sail to polars calibrated logs do matter, for 99.99% of us looking at the sails and telltales will always do better than looking at dials.